The invention relates to the field of random access communication networks, and more particularly to managing communications within such networks.
In certain networks of the type cited above, for example networks including slotted ALOHA access, communication terminals can transmit their messages only during times authorized by the network.
To be more precise, when it first accesses the network, a requesting terminal must transmit to the base station signals representing a preamble defining a request for access with a view to transmitting a message. To this end, it transmits the preamble with a signature selected at random from N signatures (N=16 in the case of a UMTS network, for example), in a specific random access channel (RACH), and in an access time slot of predefined width. In a slotted ALOHA type network the preamble has a duration equal to 4096 chips, for example, and the width of a time slot is equal to 5120 chips (which corresponds to 1.3 ms).
Each terminal has a map of the access slots during which it is authorized to send its preambles. This map, which is configurable, is broadcast periodically over the whole of the coverage area of the network, to all the terminals situated in that area. Moreover, the time reference of the base station is broadcast periodically by the network over the whole of the coverage area to all of the terminals situated in that area over a dedicated synchronization channel.
The requesting terminal can send the message associated with the transmitted preamble only if said preamble has been acknowledged by the network, more precisely by the base station. If the requesting terminal has not received an acknowledgement message within a predefined and configurable acknowledgement time, it sends another preamble in an access slot. The number of preambles that can be sent consecutively and the period with which the preambles are sent are predefined and configurable. They are broadcast periodically by the network over the whole of the coverage area to all the terminals situated in that area.
A preamble is said to be acknowledged when the base station has determined the presence of the terminal that sent it and the terminal has received an acknowledgement message from the base station within the acknowledgement time cited above.
In the above type of network the time reference (or base) that is broadcast, for example by a satellite, is received by the various terminals present in the coverage area of that satellite, with a delay relative to a reference that depends on their respective positions in the area. Each terminal must lock its timebase onto the time reference of the network. Now, because of the spread of the propagation times between the satellite and the receiver modules of the terminals, the latter are not synchronized with each other. The spacing between the authorized access slots is therefore selected to define a guard time.
If the spread of propagation times is less than or equal to the guard time, the preambles sent by the various terminals over different access time intervals are received by the base station within time slots that correspond to the access time slots without risk of offsetting and/or overlapping.
On the other hand, if the spread of the propagation times is greater than the guard time, the preambles sent by different terminals over different access time slots are received by the base station offset relative to the receive windows associated with the access time slots. This can happen if the coverage area is enlarged, for example in the case of “spot satellite” type coverage. The offset may be so great that the preambles may be received out of order. For example, in the case of a European spot coverage, the spread of propagation times over round trip paths concerning satellites at an elevation from 20° to 40° can reach 13 ms, which corresponds to an offset of approximately ten access time slots. In the case of a national spot coverage, this spread can reach 4 ms, which corresponds to an offset of approximately three access time slots.
These offsets are liable to increase the risk of collision between preambles, i.e. the probability that the base station receives more than one preamble in the same access time slot. In the above example, this is reflected in a change from slotted ALOHA type access to ALOHA type access, for the receive part, and consequently in a reduction of access capacity. Moreover, because of these offsets the receive time windows of the base station, which are locked to its time reference and of the same width as the access time slots, are no longer suitable.
To eliminate this problem, and more precisely to take account of the spread of propagation times, it has been proposed to define receive time windows extending over a plurality of access time slots. This solution has a number of drawbacks, however. A plurality of preambles received in the same receive time window might not be distinguished although they do not overlap (and therefore there is no collision). Moreover, because of the increased width of the receive time windows, the samples that constitute the signals defining the preambles received in a receive time window must be stored in a buffer for longer, which introduces an additional processing delay. Finally, the calculation time is increased, with no possibility of parallel calculation.